ocaml: phase 5.1 mat_mul.ml baseline (3x3 row-major matrix multiply, sum = 621)

Triple-nested for loop with row-major indexing:

  for i = 0 to n - 1 do
    for j = 0 to n - 1 do
      for k = 0 to n - 1 do
        c.(i * n + j) <- c.(i * n + j) + a.(i * n + k) * b.(k * n + j)
      done
    done
  done

For 3x3 matrices A=[[1..9]] and B=[[9..1]], the resulting C has sum
621. Tests deeply nested for loops on Array, Array.make + arr.(i) +
arr.(i) <- v + Array.fold_left.

50 baseline programs total — milestone.

lib/ocaml/baseline/expected.json

@@ -22,6 +22,7 @@
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "mat_mul.ml": 621,
   "json_pretty.ml": 24,
   "kadane.ml": 6,
   "lambda_calc.ml": 7,